Compressors of this type, which are used in particular in turbojet engines, are constituted by a rotor which instead of comprising a succession of separate disks stacked one after another, comprises a single drum for receiving the series of blades of different stages.
Conventionally, the single drum has recesses or retention grooves machined therein, in a disposition that leaves gaps between adjacent stages for receiving the blades of the stator stages that are secured to a fixed portion presenting a casing.
The blades are usually secured individually to the drum in a corresponding number of housings that are regularly distributed and of shape that is determined to be complementary to the shape of the blade root so as to prevent the blade from moving radially, for example by providing fastening of the dovetail type. To prevent the root of a blade moving in translation relative to its housing, in particular in the axial direction, there is generally provided a respective blocking system for each blade constituted by a ball, a pin, a clip, a flange, a spacer, etc.
The rotor of a compressor to which the invention applies comprises a metal drum of the single-block type made by machining a rough forged piece which is fitted with a downstream stiffener needed to damp radial deformations, i.e. to enable the drum to withstand stresses, in particular radial compression stresses that are exerted while the turbomachine is in operation.
The stiffener is formed by a radial protrusion constituting an inwardly-directed radial projection (i.e. projecting towards the longitudinal axis), thereby forming a zone presenting sufficient radial thickness to provide the necessary strength, this zone constituting a portion having a bore of diameter that is generally smaller than in the remainder of the drum.
Such drums are usually made of a titanium-based alloy which withstands the thermal and mechanical stresses to which turbomachine rotors are subjected.
In aviation applications, where manufacturers are constantly attempting to reduce weight, this type of blade-carrying drum is made with small thickness in the radial direction, whether in the zones for retaining the blade roots or in the zones corresponding to the recesses.
Nevertheless, in spite of such a shape of small thickness, it is necessary to begin by making a forging that is relatively thick, with this large thickness being due in particular to the downstream portion that contains the stiffener. Obtaining such a forging therefore requires a large quantity of raw material to be used, giving rise to large costs in terms of raw materials and subsequent machining, with material being lost during machining in the form of swarf.
Another technique serves to avoid the need to have recourse to a thick forging: it consists in welding on a separate piece for forming the stiffener, in particular by electron beam welding. Nevertheless, that technique requires both that an additional piece be made by forging and then machining, and that a lengthy and expensive welding operation be performed.